1. Field of the Invention
The present invention relates to image forming apparatuses forming an image with electrophotography using light from a light source reflected from an original, such as a copying apparatus and a facsimile apparatus, and more particularly, to an image forming apparatus capable of controlling the amount of light of the light source.
2. Description of the Related Art
An image forming apparatus forming an image by irradiating an original with light from a light source and directing light reflected from the original to a photoreceptor includes a detector for detecting the amount of light (hereinafter referred to as a "light amount detector") reflected from the original. The image forming apparatus controls the amount of light of the light source based on an output signal from the light amount detector. By controlling the amount of light of the light source based on the amount of reflected light as described above, a constant image density can be obtained independent of the density of the original. In controlling the amount of light of the light source based on the amount of light reflected from the original, the characteristics of the light amount detector formed of a photodiode or the like sometimes change over time, causing output signals for the same amount of input light to vary. If no adjustment is made to such output signal variation, the image density cannot be maintained appropriately. Manual adjustment of the level of the output signal of the light amount detector is troublesome.
FIG. 8 shows a structure of a light amount control portion of a conventional image forming apparatus disclosed in Japanese Patent Laying-Open No. 4-347877. As shown in FIG. 8, light of a light source 81 is directed to an adjustment pattern 82. A control portion 85 controls an electrically controlled volume 84 so that an output signal from a light amount detector 83 in receiving light reflected from adjustment pattern 82 is within a predetermined range. By being thus structured, the image forming apparatus can adjust the level of the output signal of light amount detector 83 receiving light reflected from adjustment pattern 82 of a constant density to a predetermined value even when the characteristics of light amount detector 83 change over time.
However, no conventional image forming apparatus took into consideration incidence of light of the light source on the light amount detector after reflection from a member other than the original, or direct incidence of light of the light source on the light amount detector. Such light other than light reflected from the original is incident on the light amount detector both in a copy mode and in an adjustment mode to act as bias light to the total amount of light received by the light amount detector. The amount of the light other than light reflected from the original changes according to the amount of light of the light source. Therefore, when the amount of light of the light source changes due to attachment of scattered toner, paper pieces, or dust to the light source, it is not possible to correct variation of the level of the output signal of the light amount detector influenced by the light other than the light reflected from the original only by adjusting the output signal of the light amount detector to be within a predetermined range in the adjustment mode, making it impossible to maintain a favorable image formation state.